1. Field of the Invention
The present invention relates to a fired-type conductive paste containing glass frit and a conductive powder comprising silver as a main component and used for forming a solar cell electrode.
2. Description of the Related Art
Conventionally, an ordinary solar cell device is provided with a silicon semiconductor substrate, a diffusion layer, an antireflective film, a rear electrode and a front electrode (hereunder sometimes called a “light-receiving electrode”). When forming the front electrode in particular, the electrode is formed by screen printing, stencil printing or the like, using a conductive paste made by mixing conductive particles composed mainly of silver with glass frit, an organic vehicle, etc.
As one example, in the crystalline silicon solar cell shown in FIG. 1, a diffusion layer 3 is formed in the front surface (light-receiving surface) area of a p-type crystalline silicon substrate 4, which is formed with a concave-convex surface structure called a textured structure. The diffusion layer 3, which is formed by diffusing an impurity such as phosphorus (P) into the semiconductor substrate 4 from the light-receiving surface thereof, is a region exhibiting the opposite conductivity type from the semiconductor substrate 4 (in the present example, the opposite conductivity type is explained as n-type). The n-type diffusion layer 3 is formed for example by placing the semiconductor substrate 4 in a diffusion furnace, and heating it in phosphorus oxychloride (POCl3) or the like. An insulating antireflective film 2 is formed from silicon nitride, silicon oxide, titanium oxide or the like on this diffusion layer 3 to provide an antireflective function while at the same time protecting the solar cell device. In the case of silicon nitride (hereunder, “SiN”) for example, the film is formed by plasma CVD or the like using a mixed gas of silane (SiH4) and ammonia (NH3). The antireflective film 2 is formed with a thickness of about 5 to 100 nm and a refractive index of about 1.8 to 2.3, taking into consideration the difference between this refractive index and that of the semiconductor substrate 4 and the like.
Next, the aforementioned conductive paste is printed or coated in a grid form on the antireflective film 2 by screen printing or the like, and fired at about 500 to 900° C. to form a front electrode 1. Normally, electrical contact between the front electrode 1 and the n-type diffusion layer 3 is achieved when antireflective film 2 is melted by the action of the glass frit in the conductive paste and removed during firing. This is commonly called “fire-through”.
A rear electrode 5 is formed on the rear side of the semiconductor substrate 4 together with a highly concentrated p-type BSF (Back Surface Field) layer doped with aluminum or the like.
To achieve proper fire-through, glasses having a good solubility with the antireflective film 2 have been preferably used as the glass frit in the conductive pastes. Among them, especially glass containing lead oxide has often been used for the glass frit in conventional conductive pastes for forming front electrodes because the glass softening point is easy to adjust and the glass provides good adhesiveness with the substrate (adhesive strength), allows for relatively good fire-through and results in superior solar cell characteristics.
For example, lead borosilicate glass frit is used in the silver pastes for forming solar cell electrodes described in Japanese Patent Publication Nos. 11-213754 A, 2001-093326 A and 10-326522 A, while Japanese Patent Publication No. 2001-118425 A describes lead borate glass frit in addition to lead borosilicate glass frit.
Regarding the aforementioned fire-through, however, problems with variation in adhesive strength and failure to obtain stable ohmic contact between the front electrode 1 and the n-type diffusion layer 3 of the semiconductor substrate 4 have occurred when the front electrode 1 does not penetrate through the antireflective film 2 due to variation in the effect of the glass frit and the like when the front electrode 1 is fired. Insufficient ohmic contact can cause loss during output, resulting in lower conversion efficiency of the solar cell and a decline in the current-voltage characteristics.
As described in paragraph [0004] of Japanese Patent Publication No. 10-326522 A, paragraph [0017] of Japanese Patent Publication No. 2004-207493 A, etc, meanwhile, there has been known another problem in which excessive fire-through may also produce inferior voltage characteristics. Since the antireflective film 2 can be no more than about 5 to 100 nm thick as described above, if the front electrode 1 penetrates through the antireflective film 2 and then through the n-type diffusion layer 3 below to intrude into the semiconductor substrate 4, the p-n junction may be broken, and the fill factor (“FF”) obtained from the current-voltage characteristic measurements may be adversely affected. Such penetration may become more likely and harder to control if in the future the n-type diffusion layer 3 is made still thinner in an effort to improve efficiency.
FIG. 2 shows the interface between a front electrode and a semiconductor substrate of a commercial solar cell substrate as seen through a transmission electron microscope (TEM). Lead glass is used in the front electrode of this commercial solar cell. In FIG. 2, a lead glass layer 6 containing a silver component from the conductive paste is present between the front electrode layer 1a and the SiN layer 2, which is an antireflective film, and part 7 of this glass layer penetrates through the SiN layer 2 to contact a silicon substrate (or n-type diffusion layer) 4, but in part 8 there is too much fire-through, and the glass can be seen as protrusions intruding deeply into the interior of the semiconductor substrate 4.
As a separate issue, increased environmental awareness in recent years has led to a desire for a switchover to lead-free materials and parts in solar cells. Alternative materials and parts are therefore being developed that will provide ease of adjusting the softening point of the glass, good adhesiveness to the substrate (high adhesive strength) and good fire-through, as in the case of the conventional lead glass, with the aim of providing superior solar cell characteristics.
For example, attempts have been made to form front electrodes using zinc borosilicate glass frit in Japanese Patent Publication No. 2001-118425 A, bismuth borosilicate and zinc borosilicate glass frits in Japanese Patent Publication No. 10-326522 A, borosilicate glass frit in Japanese Patent Publication No. 2008-543080 A (Japanese translation of WO 2006/132766) and zinc borate glass frit in Japanese Patent Publication No. 2009-194121 A. However, the research of the present inventors has shown that even using such lead-free glass, fire-through is sometimes difficult to control, including cases of insufficient fire-through, failure to achieve ohmic contact, or, as in FIG. 2, excessive fire-through such that part of the front electrode intrudes deeply into the semiconductor substrate.
On the other hand, tellurium glass is known as a glass for use in fluorescent display tube sealing applications (Japanese Patent Publication No. 10-029834 A) and optical fiber material applications (Japanese Patent Publication No. 2007-008802 A). In general, tellurium glass is known to have a low melting point, to be highly durable and to easily dissolve silver in solid solution, but it also has extremely low reactivity with silicon oxide, and since silicon-type antireflective films have been popular in recent years, there has been little interest in tellurium glass for forming the front electrodes of solar cells.